Answer: c. Going through one condition may influence how participants behave in another condition. Randomizing the order helps minimize the impact of these influences.
To illustrate how this would work, let's think of an experiment where the subjects have to perform a physical and a mental test. Performing the physical test before the mental one might lead you to do worse on the mental one out of tiredness. Therefore, if all your subjects did the physical exams first, your data on the mental exam would be unfairly low. The opposite order might affect in the same way the results of the physical exam.
A way to minimize these effects is to randomize the order, so that the effect of both orders is reduced in the final results.